1. Field of the Invention
The present invention relates to a method of controlling an induction motor by bringing .gamma.-.delta. coordinate axes which are reference coordinate axes of a control system configuration into agreement with d-q coordinate axes where the d-axis represents the direction of the vector of secondary magnetic fluxes of the induction motor and the q-axis represents an axis perpendicular to the d-axis.
2. Discussion of the Background
Processes for estimating the speed of an induction motor using the induced electromotive force (induced voltage) are employed in industrial applications for the purpose of making the V/f control highly accurate and simplifying calculations for the speed-sensor-less vector control because the processes are physically obvious from the fact that an induced electromotive force is generated when the rotor rotates and also because the processes are able to calculate the rotational angular velocity of secondary magnetic fluxes with a relatively small amount of calculations.
Examples of conventional processes include [1] "Study of making speed-sensor-less vector control highly responsive", 1994 Electric Society National Conference No. 655, and [2] "Speed-sensor-less vector control in view of weak field region", 1994 Electric Society National Conference No. 656. The former process pays attention to a voltage model of the q-axis of a d-q coordinate system and proposes a simple disturbance estimator for estimating a q-axis induced electromotive force using a q-axis voltage command value as an input and a q-axis current as an output. The simple disturbance estimator estimates a speed by multiplying the estimated q-axis induced electromotive force by coefficients representing a motor constant and a flux command value. The former process is an improvement over a process that uses the output of a q-axis current controller as an estimated speed. The former process achieves separation between a speed control system and a current control system by introducing the estimation of an induced electromotive force, and is successful in improving the speed control response. The latter process estimates an induced electromotive force from a measured voltage and a measured current using a voltage equation of the induction motor, estimates a power supply angular frequency from the estimated induced electromotive force and an estimated value of secondary magnetic fluxes, and subtracts a command value for a slip frequency from the estimated power supply angular frequency to estimate a speed.
The above conventional processes basically use voltage and current equations in the d-q coordinate system of an induction motor, i.e., ##EQU1##
where .epsilon.d: a d-axis component of the induced electromotive force, .epsilon.q; a q-axis component of the induced electromotive force, .PHI.rd: a d-axis component of the secondary magnetic fluxes, .PHI.rq: a q-axis component of the secondary magnetic fluxes, Isd: a d-axis component of the stator current (excitation-related current), Isq: a q-axis component of the stator current (torque-related current), usd: a d-axis component of the stator voltage, usq: a q-axis component of the stator voltage, Rs: a stator resistance, Lm: mutual inductance, Lr: rotor self-inductance, Ls: stator self-inductance, .delta.=1-Lm.sup.2 /(LsLr), .omega.*: a power supply angular frequency. The conventional processes estimate an induced electromotive force according to the equation (1), and estimate a speed according to the equation (2).
Since a current differential term in the right-hand side of the equation (1) is omitted, the conventional processes fail to estimate a correct induced electromotive force while the current is varying. If the current differential term is not omitted, then noise contained in a measured current value is amplified. The literature [1] proposes a process of equivalently canceling a low-pass filter used to output an estimated speed and a current differential term. The literature [1] assumes an ideal vector control state, estimates only a q-axis induced electromotive force, and estimates a speed using the estimated q-axis induced electromotive force. However, there is a possibility that the ideal vector control state may collapse due to a change in the induction motor constant and errors of measured current and voltage with respect to actual current and voltage. The literature [1] is silent about how to compensate for such a collapse of the ideal vector control state. The literature [2] proposes the following equation (3) in order to compensate for a collapse of the ideal vector control state: EQU .omega.*=sqn(.epsilon.q)(.vertline..epsilon.q.vertline..PHI.rd-K.PHI.rd*.ep silon.d) (3)
In a steady state, the existence of .epsilon.d signifies the existence of .PHI.rq as can be seen from the equation (2), and indicates that the ideal vector control state has collapsed. Specifically, the .gamma.-.delta. coordinate axes which are reference coordinate axes of the control system configuration and the d-q coordinate axes are displaced from each other. Axis-displacement compensation is carried out by eliminating .epsilon.d in the second term of the right-hand side of the equation (3). However, the equation is contradictory in that .omega.* is necessary to estimate the induced electromotive force used in the equation (3) and .omega.* is necessary to determine .omega.*. In order to actually perform software-implemented calculations of the equation, .omega.* at a preceding time in a discrete system is used. If .omega.* varies quickly, then the speed response tends to be delayed.